1. Field of the Invention
This invention relates to chassis dynamometers for simulating the inertia and road load forces encountered by motor vehicles under anticipated driving conditions and more particularly to such dynamometers which employ an eddy current brake as the power absorbing unit.
2. Description of the Prior Art
Test apparatus in the form of dynamometers is widely used for testing motor vehicles in place. Since the test vehicles are not moving over a road bed, the dynamometer must simulate certain forces normally associated with actual vehicle operation. These parameters include forces associated with inertia (related to the mass or weight of the vehicle) and road load forces (related to the velocity of the vehicle). The vehicle engine (or its braking system) must overcome inertial forces in order to accelerate or decelerate the vehicle. In addition, the engine must overcome breakaway frictional and rolling frictional forces (i.e., road/tire friction) as well as windage forces (i.e., drag forces caused by air passing over the vehicle). These latter forces are commonly referred to as road load (RL) forces and may be represented by the formula: EQU RL=A+BV+CV.sup.2 +DW
where A represents the vehicle constant load coefficient (e.g., effects of breakaway force), and, B and C represents the vehicle load coefficient dependent on velocity and velocity squared (e.g., windage), and D represents the grade coefficient (e.g., slope of the grade). It should be noted that a load coefficient based on velocity cubed may be added if desired). V represents the vehicle velocity and W represents the vehicle weight.
The purpose of the dynamometer is to impose those forces on the vehicle which the vehicle would incur during actual operation on a road. Such dynamometers include a roll (or a pair of rolls) for engaging the driven wheel (e.g., motorcycle) or wheels (e.g., automobile) of the vehicle being tested. It should be noted that where 4 or 6 wheel drive vehicles are to be tested the dynamometers will include 2 or 3 pair of rolls. The roll or rolls are supported by a shaft journaled in bearings mounted on a frame.
Typically a power absorbing and supplying unit such as an electric motor or a power absorber per se such as an eddy current brake, friction brake or hydrokinetic brake carried by the frame, is coupled to the roll for absorbing power from the roll. The roll in turn applies a retarding force to the surface of the vehicle wheel (e.g., tire) to simulate the road load and inertial forces. Inertial forces can be simulated by an electric motor during acceleration as well as deceleration but can only be simulated by a power absorber/brake during acceleration only because such braking units only absorb and do not supply power. Generally (and often when electric motors are used) a large part of the inertial forces are simulated by selectively coupling the roll to one or more mechanical flywheels. The combined inertia of the flywheels, the roll and other rotating components exert a force on the vehicle wheels proportional to the acceleration (or deceleration) of the vehicle wheels. Thus, the engine is required to expend as much power in accelerating the roll as it does in overcoming the vehicle inertia during actual road acceleration. Road load and inertial forces which must be simulated by the dynamometer may be calculated from the formula: ##EQU1##
wherein I represents the simulated inertia, dv/dt the derivative of velocity with respect to time (or the acceleration of the vehicle).
While electric motors (power supplying and absorbing units) increase the versatility of a dynamometer they also significantly increase the cost. For that reason, dynamometers equipped with power absorbers which provide braking only are in considerable demand for mass vehicle emission testing stations, mandated by the U.S. Environmental Protection Agency ("EPA"), as well as for facilities which must provide the necessary repairs so that rejected vehicles can pass a renewed emissions test. Both friction brake and eddy current brake dynamometers are typically less expensive to manufacture than dynamometers equipped with electric motors or hydrokinetic absorbers. However, friction brakes are subject to wear and thus require considerable maintenance. Eddy current brake dynamometers do not suffer the wear problems associated with friction brake machines and are particularly cost effective for EPA testing and repair applications.
Dynamometers utilizing an eddy current brake (or other power absorbing units) are designed with a base or minimum inertia i.e., the smallest vehicle inertia which can be simulated and a maximum inertia, i.e., the largest inertia which can be simulated. The minimum dynamometer inertia is made up of the inertia of the roll or roll set, the inertia of the rotating component of the absorber and a trim inertia. The term roll or roll set inertia, as used herein, shall mean the rotational inertia of the roll or roll set which engage the vehicle wheel(s) plus the inertia of minor inertia contributing auxiliary components such as the shaft(s), couplings and gears or belts, where used, which are permanently connected to the roll. The inertia of such auxiliary components is less than that of the roll or roll set.
The trim inertia in a typical dynamometer utilizing an eddy current brake (or other power absorbing unit) is made up of a fixed flywheel, i.e., a mechanical flywheel permanently connected to the roll. As an example, an eddy current brake dynamometer designed to test motor vehicles weighing 2000 pounds or more would include a roll set inertia of say 300 pounds, a braking unit inertia (i.e., the rotor inertia) of say 200 pounds and a fixed flywheel inertia of 1500 pounds. Additional flywheels, clutched to the roll shaft, generally make up the difference between the dynamometer's minimum inertia (e.g., 2000 pounds) and its maximum inertia (e.g., 5000 to 6000 pounds). It should be noted that the braking unit per se may be used to simulate the difference between the base and maximum inertia during acceleration only as pointed out above.
Eddy current brakes comprise a rotating wheel made of a suitable electrically and magnetically conducting material with sufficient strength to withstand the centrifugal and other forces encountered. Stationary field coils are positioned adjacent the rotor for generating an exciting magnetic field which in turn induces eddy currents in the rotating rotor. Eddy currents generate the secondary high density magnetic field in a gap between rotor and coils which causes the absorber to apply a braking torque to the roll and to the vehicle tire in contact with the roll. The amount of braking torque is determined by the density of the magnetic field which is controlled by the current supplied to the field coils.
Typical eddy current brakes are designed to minimize the rotor mass so that energy is not wasted in accelerating or decelerating extra mass.
We have discovered, contrary to the prevailing view, that there are advantages to increasing the rotor mass of an eddy current brake so that the rotor makes-up the dynamometer's trim inertia. The resulting eddy current brake/trim inertia simulating unit provides several important advantages. First, the combined unit eliminates the need for a separate trim inertia flywheel (and bearings) with its attendant extra costs (i.e., about 5% of the cost of dynamometer capable of simulating inertias of 2000 to 5500 pounds). Second, and much more obscure, the large rotor mass accommodates higher power absorptions because of the much larger heat sink and the attendant increased area for heat dissipation. Third, the large rotor mass surprisingly requires lower magnetic field strength and lower power consumption and much lower magnetic saturation of the rotor iron as compared with a conventional eddy current brake having a comparable short term braking capacity. The net result is the elimination of an expensive trim flywheel and higher power/torque absorption levels.